This invention relates to a spectroscopic system having an improved spectral signal-to-noise ratio and a simpler configuration involving fewer mechanical parts than current grating-based instruments.
At a minimum, a spectrometer (e.g., a Raman, fluorescence, absorption or reflectance spectrometer) includes a light source, a photo-detector and a wavelength-selecting device. A wavelength-selecting device is often called a spectroscope. Depending on the application, the sample can be inserted between the source and the spectroscope or between the spectroscope and the detector.
There are different types of spectrometers. One type of spectrometer is a wavelength-scanning instrument. A wavelength-scanning instrument acquires one wavelength band at any given time. Examples of this type of device are monochromators, tunable interference filters (tunable through various techniques such as mechanical tilting, thermal variation, and opto-acoustical variation) and discrete-filters-spinning-wheel instruments. A discrete-filters-spinning-wheel instrument includes a wheel with a number of discrete filters. The wheel is oriented substantially perpendicular to the path of an optical beam so that the axis of rotation of the wheel is substantially parallel to the optical beam and so that by rotating the wheel one can alternately bring each of the discrete filters into the path of the optical beam. In other words, only one of the discrete narrow bandpass filters is in the path of the light beam at a time.
There are two common methods of sampling and acquiring a radiation signal using a monochromator and at least one of these two common methods is also applicable to tunable interference filters and discrete-filters-spinning-wheel instruments. The first method, termed continuous-scan, controls the wavelength-selecting device to continuously vary the output wavelength. In a monochromator-based instrument, for example, the continuous-scan method smoothly varies the grating angle as a function of time. The continuous-scan method acquires and digitizes the signal while the grating is rotating. One usually performs a rotation cycle, i.e., a cycle involving moving a grating through the grating's range of motion (less than 90 degrees), a number of times. One then averages the acquired data to improve the resulting signal-to-noise ratio. In other words, such a monochromator-based system needs to slow down the rotation of the grating, bring the grating to a stop and start rotating the grating in the opposite direction and the system typically needs to repeat this process many times.
The second method, termed step-and-scan, rotates the grating/filter (in a tilt-tuned instrument) step-wise to a new location and lets the grating/filter settle before starting data acquisition. The method integrates (averages) the signal while the grating is at rest.
Current wavelength-selecting devices, such as monochromators, have some inherent drawbacks:    1. Measurements are usually slow. The inertia of the moving parts (gratings, gear-reduction mechanisms, etc) usually limits the speed of rotation of the moving parts. For the continuous scan method, significant delay occurs as a result of the need to let the grating or filter come to a stop and start moving in the opposite direction with precision and without causing large vibrations. For the step-scan method, significant delay occurs as a result of the need to let the grating or filter settle to within a certain permissible angular window before starting data acquisition.    2. For monochromator and tilting filter instruments, the precision of the rotary stage at least partially determines wavelength repeatability. The rotary stage of such instruments is typically not precise enough for most high-SNR or low-noise applications, requiring high spectral repeatability such as quantitative spectroscopy measurements Other scanning systems (e.g., thermal and opto-acoustic systems) have their own inherent limitations in regard to their ability to reliably and accurately reproduce a specified wavelength.    3. Systems designed for high SNR applications are expensive due to a need for high-precision mechanical and electrical components.    4. Current wavelength-selecting systems are highly sensitive to mechanical vibrations.
For applications requiring high SNR(i.e., most applications today, especially quantitative applications), one typically employs modulation and bandwidth-narrowing techniques. These techniques commonly involve modulating the radiation signal so that one can apply electrical bandwidth-narrowing techniques, such as band-pass filtering and phase-locking techniques, to the resulting signal.
To expand on this last point, in a grating-based spectrometer that does not use modulation, one can tilt the grating to select a particular narrow band of wavelengths and then one integrates the signal over time. Such a method has the disadvantage that the sources of noise regardless of their respective frequencies add on to the resulting signal, negatively impacting the signal-to-noise ratio (SNR). However, by applying modulation at a frequency distinct from the frequencies of most sources of noise, one can increase the resulting SNR.
Spectroscopy practitioners commonly use a mechanical chopper to modulate the radiation arriving at the photo-detector. A mechanical chopper works by mechanically “chopping” (or blocking) the beam path periodically at a predetermined frequency.
Light modulation using such a device has some disadvantages:
                1. A mechanical chopper adds complexity to a spectroscopic system. Besides the chopper wheel itself (and other components needed to support rotation of the wheel), the device needs its own electromechanical and electrical components to drive and control the rotation of the chopper wheel and to synchronize the chopper wheel with an associated wavelength-selecting instrument. Thus, modulation using a mechanical chopper can add significant cost to a spectroscopic system.        2. Mechanical choppers are typically bulky and difficult to miniaturize.        3. Measurement becomes slower, because the wavelength-scanning device needs to “wait” for the light chopper to produce enough turns/modulations before proceeding to the next wavelength segment. The slower the modulation frequency, the longer the mechanical-chopper-modulated system needs to obtain a given number of signals. This type of chopping device is usually limited to only a few hundred hertz.        4. Modulation using a mechanical chopper does not eliminate the low frequency (close to DC frequency) stability error, such as those caused by temperature variations or power drifts.Thus, a need remains for inexpensive spectroscopic systems that have an improved spectral signal-to-noise ratio and a simpler configuration involving fewer mechanical parts than current grating-based instruments.        